Enhanced schlieren imaging applied in heat and air jet visualizations: a wave propagation-based model

Refractive index variations caused by temperature or pressure gradients in transparent fluids are invisible to the naked eye. Schlieren effect reveals this variation using refraction and the knife-edge method. High contrast schlieren images are important in the analyses of fluid flow, gas density, shockwaves, heat transfer, flames, ballistics, leak detection and other applications. The neglect of physical or wave theory in schlieren technique leads to erroneous results in some circumstance. Specifically, a study had mathematically shown that illumination is fairly uniform over large part of the field but suddenly increases at the edge and is fairly appreciable for some way outside the actual physical boundary of the aperture. This bright edge is noticeable in all schlieren systems whereas a geometrical optics would lead to a uniformly illuminated field. Geometric ray-tracing codes are useful for optical design, but they cannot describe the key role of diffraction in the formation of schlieren image. In this study, a wave propagation-based model of the schlieren technique is proposed. Compared to the ray optics approach, the proposed model provides valuable insights and visualization of fluid flow dynamics. Some predictions of the model will be confirmed through experimental demonstrations. Setup parameters are also optimized resulting in enhanced resolution of schlieren images.